Improvements in noise, vibration and harshness (NVH) in automobile systems, significantly in the area of powertrain systems including engines, transmission and other driveline components, are always being sought. One readily identifiable source of NVH can be attributed to torsional torque disturbances induced by relatively high compression and expansion forces acting upon the engine's pistons during a start-up sequence prior to ignition (i.e. engine cranking). Since engine cranking historically has been a relatively infrequent and temporally limited event—typically one per driving cycle—improving NVH from other sources has been given higher priority. However, the emergence of automobiles, particularly hybrid electric vehicles, that enable engine shut-down during periods of vehicle operation when it is not required (e.g. traffic signal idle) has required significantly more frequent engine starts during a normal driving cycle. The engine starts are effected without conventional operator interaction (i.e. rotation of ignition lock cylinder). That is to say, engine starts may be effected, for example, subsequent to the operator releasing the vehicle service brake pedal and depressing the vehicle accelerator pedal. Additionally, engine starts may be effected to satisfy various vehicle demands including, for example, battery state-of-charge maintenance and exhaust catalyst thermal management. While such technological advances can significantly and favorably impact overall vehicle fuel economy and tailpipe emissions, a relatively new source of potentially objectionable NVH to the operator has been created thereby.
Engine starting sequence NVH may be improved in some engines through advanced valvetrain controls. For example, single cylinder engine starts wherein all cylinders save one are mechanically deactivated may show NVH improvements. Likewise, cam phasing may be employed to reduce compression pressures during engine starts with the same improvement objective. However, such solutions require additional valvetrain complexity and expense and are not implementable in engines having conventional valvetrain hardware.